1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus for forming a toner image on a sheet in an electrophotographic method.
2. Description of Related Art
In the art of laser printer, in order to make a compact printer, various improvements have been made on a sheet passageway therein. For example, Japanese Patent Laid Open Publication No. 1-317939 discloses that sheets are fed from a sheet cassette which is set along a bottom of a printer body toward a front side of the body (a side where an operation panel is disposed), that each fed-out sheet receives an image while being conveyed upward in the front side and that the image-formed sheet is ejected toward a rear side of the body onto a tray disposed on an upper side of the body. The sheet passageway in the printer is formed substantially along the inner wall of the printer body, and this contributes to the compactness of the printer. However, this printer has a problem in sheet ejection. Image-formed sheets ejected from the printer are collected on the rear side of the printer body, that is, the sheets are collected in a far side from an operator, which is inconvenient to the operator.
Focusing on a sheet feeding mechanism, conventional laser printers and electrophotographic machines generally adopt a mechanism wherein a sheet is first fed out of a stack of sheets horizontally and then conveyed upward guided by a plate (refer to Japanese Patent Publication No. 2-31465). The horizontal sheet feed-out of this mechanism makes it difficult to make a compact image forming apparatus. Also, there is much possibility of occurring sheet jamming in the apparatus.
Now regarding a sheet feeding section for supplying sheets to an image transfer section, an image forming apparatus such as a laser printer or an electrophotographic machine conventionally employs a detachable sheet cassette containing a stack of sheets and/or a manual sheet feed tray disposed on an outside frame of a body of the apparatus.
A sheet cassette used as the sheet feeding section is exclusive for a size of sheets, and in order to supply different sizes of sheets, it is required to prepare several sheet cassettes. However, this is costly. Further, in order to make it possible to supply sheets which cannot be placed in any of the prepared cassettes, the manual sheet feed tray must be provided. Generally, the manual sheet feed tray is space-consuming, that is, the tray is protruded laterally from the body of the apparatus.
Incidentally, recently an image formation cartridge is used in an image forming apparatus, and U.S. Pat. No. 4,786,938 proposes an image formation cartridge containing not only a photosensitive member but also transferring means. The photosensitive member and the transferring means face each other with a sheet path in-between. In this type of image formation cartridge, the surface of the photosensitive member does not need to be protruded from a housing of the cartridge for image transfer. Thus the photosensitive member is well protected by the housing.
However, when a roller is used as the transferring means in the image formation cartridge, there occurs a problem in treating sheet jamming. In the cartridge, the transfer roller is always in contact with and pressed against the photosensitive member. When a sheet is stuck between the transfer roller and the photosensitive member and also stuck between other transporting rollers, if the image formation cartridge is displaced for treatment of the jammed sheet, the sheet may be torn up or the photosensitive member may have a scratch.